Houston: Donald Trump has taken firm control of the race for the Republican presidential nomination with his third straight victory, in Nevada. To deny him that role, strategists say, his leading rivals must quickly change the trajectory of the race and then dig in for what could be a long battle that could go all the way to the GOP convention in Cleveland.

Hillary Clinton warns of fake news

Trump in Nevada: 'I love the poorly educated'

After his win in Nevada, Donald Trump tells supporters in Las Vegas that he got votes from well educated and poorly educated people, adding "I love the poorly educated."

Strategists who have been through past nomination battles say that Senator Marco Rubio, Senator Ted Cruz and Ohio Governor John Kasich collectively have until March 15 to turn the race away from the New York billionaire. Each has a must-win test looming in his home state between now and then. But those victories alone might not be sufficient to block Trump's path.

As for Trump, the front runner hopes to suffocate his three main rivals in next week's Super Tuesday contests, particularly in primaries across the Deep South, and to knock them off with wins in each of their home states. In his Las Vegas victory speech after the Nevada caucuses on Tuesday night, Trump predicted he would secure the nomination in less than two months and taunted his top three opponents by trumpeting his high poll numbers in Florida, Ohio and Texas.

Donald Trump is looking harder to reach for his Republican rivals. Photo: Bloomberg

The first opportunity to alter the campaign could come on Thursday night, when the Republican rivals meet in Houston for their first debate since the departure of former Florida governor Jeb Bush, who quit the campaign on Saturday after finishing fourth in South Carolina.

Changing the dynamic, however, depends on whether any of them has a strategy to put Trump on the defensive. So far, Trump's rivals have focused more on one another than on him, in hopes of becoming Trump's last viable opponent. Many Republicans fear that strategy will only allow Trump to put a stranglehold on the nomination with continued victories in the coming weeks.

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"The reality is that, until the field starts to narrow, it's going to be very, very hard to take him out," said Katie Packer, deputy campaign manager for Mitt Romney's 2012 campaign and the leader of an anti-Trump super PAC. "I think people need to step up and start taking on Trump. Front runners don't just stumble. People trip them."

After losing the Iowa caucuses to Cruz on February 1, Trump has piled up victories in New Hampshire, South Carolina and Nevada. He heads into next week's Super Tuesday contests in 11 states with the anti-Trump vote still splintered among his rivals.

Given Trump's string of successes, his rivals must demonstrate that they can beat him, not just finish ahead of each other.

"You have to win," Stuart Stevens, chief strategist for Romney in 2012, said. "Winning transforms a candidate, and losing transforms a candidate. You're not the same person after you've won a bunch of races, and you're not the same after you've lost a bunch of races."

Winning states does more than change perceptions. It's the key to amassing delegates. From here forward, the campaign shifts from a battle for momentum to the trench warfare of gathering delegates.

Trump has taken hold of the delegate race in large part by winning all 50 that were at stake in South Carolina on Saturday. The longer the field remains divided, the better Trump's opportunities to maintain or enlarge that lead.

David Wasserman, of the Cook Political Report, updated his delegate charts on Wednesday morning and concluded that Trump is on pace or slightly ahead of what he would need to win the 1237 delegates needed to secure the nomination. Rubio and Cruz are behind their pace to achieve that.

Only about 5 per cent of the total delegates have been awarded so far. But over the next three weeks, they will come in bunches, with 595 at stake on Tuesday and 368 more through March 12.

Donald Trump's pathway to the Republican nomination is looking more certain. Photo: Bloomberg

At that point, the rules shift in what could be a game-changing fashion. Between now and March 14, delegates will be awarded proportionally. Starting on March 15, states will be allowed to award delegates on a winner-takes-all basis. On that day alone, Florida, Illinois, Missouri and Ohio, with a combined 292 delegates allocated in that way, are among the states or territories with contests.

"It's important to take a deep breath here," Rubio said on Wednesday on CBS This Morning. "It's not based on how many states you win – it's based on how many delegates you picked up."

Looking ahead to the March calendar, Rubio said there were "plenty of states out there that – in the winner-take-all category – that if you win them, you more than catch up."

Over time, Rubio intends to more aggressively draw a contrast with Trump and call him out for inaccuracies or shallow policies. "We believe a majority of Republicans don't want Trump to be the nominee and will vote for an alternative," Rubio's communications director, Alex Conant, said. "As this becomes a two-person race, Marco will beat Trump head to head."

Jeff Roe, Cruz's campaign manager, argued that his candidate still has the best chance to defeat Trump, despite a loss in South Carolina.

"He's won three in a row," Roe said of Trump. "That's not lost on me." Still, he said that based on the campaign's projections of states and demographics and possible support, "the road map through March 15th will not be smooth sailing for Donald. For those that are hysterically declaring the race over should take a breath and look at the remaining 46 states."

At this point, however, none of Trump's leading rivals sees any incentive to get out of the race. All can sketch a path to the nomination, however difficult, and until they are proven wrong, they will soldier on.

Mike DuHaime, who was chief strategist for the presidential campaign of New Jersey Governor Chris Christie, said he could imagine long-odds scenarios for Cruz, Rubio or Kasich and said the psychology of believing there is a path to victory, no matter what outsiders think, will keep the candidates going.

"I have been in those rooms," DuHaime said. "As long as you believe there is a path and a chance, you keep going."

Meanwhile, Trump's insular campaign is quickly expanding into a national organisation. Workers are on the ground in states with Super Tuesday primaries and are building grassroots networks there and in other states such as Florida. Campaign manager Corey Lewandowski said Trump has lined up a "crazy travel schedule" over the next couple of weeks and brushed off any suggestion that his boss was stoppable.